There have been a variety of reactors that have been proposed to react oxygen with a hydrocarbon containing stream to produce a synthesis gas product containing hydrogen and carbon monoxide. Typical reactors are partial oxidation reactors in which the hydrocarbon species are mixed with an oxygen containing gas and are partially oxidized with the aid of a partial oxidation catalyst. Other reactors also inject steam so that the hydrocarbons can be reacted by known steam methane reforming reactions. In such a reactor, the partial oxidation reactions being exothermic provide the heat to support the endothermic heating requirements of the steam methane reforming reactions. Such a reactor is known as an autothermal reactor. Yet further reactors are multi-tubular reactors used for exothermic selective oxidation reactions for production of ethylene oxide, vinyl acetate, and other oxygenated hydrocarbons.
Reactors that are designed for partial oxidation reactions contemplate an operation in which the proportions of hydrocarbons and oxygen are selected to produce a substantially complete conversion of the hydrocarbons to a hydrogen and carbon monoxide containing synthesis gas. As such, there exists such a significant content of oxygen that autothermal ignition of the hydrocarbons is possible. Reaction of the hydrocarbons and the oxygen prior to the catalyst for any reason is particularly not desirable because it results in unwanted consumption of the reactants by full oxidation thereof resulting in a fall-off in required production rates and potential carbon deposition on the catalyst. This problem is exacerbated in such reactors because oxidation reactions are occurring directly downstream in the reaction section at high temperature and thus, combustion within the reaction section can propagate an unwanted reaction within the mixing chamber. In order to combat this problem, reactors have been designed such that the reactants, namely, hydrocarbons and oxygen are mixed in a mixing section so rapidly that they do not have time to react before a reaction section is reached containing a catalyst to promote the intended reaction.
An example of a reactor that is designed to prevent combustion of the reactants in the mixing section can be found in U.S. Pat. No. 4,865,820 that discloses a partial oxidation reactor in which the mixing chamber is provided with narrow passageways having straight throat sections in which either of the reactant streams is introduced to mix under turbulent conditions with the other of the reactant streams through orifices formed in the narrow passageways. The resultant turbulent flow has a velocity that exceeds that of any flame propagating due to flash-back from the reactor. U.S. Pat. No. 5,886,056 has provision for injecting reactant gases at high velocity through a plurality of isolated passageways in an injector manifold to reduce the residence time of the reactants within the mixing section to prevent the undesirable reaction of the reactants within the mixing section. In U.S. Pat. No. 6,471,937, hot reactant gases are introduced into a nozzle contained in a mixing chamber to produce a supersonic velocity jet that will entrain another component of a reactive mixture into the jet. Reactant mixtures are then introduced into a reaction zone. The residence time within the mixing chamber sufficiently brief that the reactants do not have time to react before entering the reaction zone.
The problem with all of such reactors is that they are not amenable to an operation in which it is not desired to completely react the hydrocarbons to a synthesis gas. For example, a catalytic partial oxidation reactor can be utilized as a pre-reformer to react higher order hydrocarbons to primarily methane. When such a reactor is used as a pre-reformer, the amount of oxygen on a volume basis that is introduced relative to the hydrocarbon feed is a fifth or less. This is to be compared to a reactor designed for complete reaction of the hydrocarbons to carbon monoxide and hydrogen in which the ratio would be a half or more. As such, devices that are described in the patents listed above and that all depend upon entrainment will not work with such a small proportion of oxygen. In any case, the mechanism of possible combustion of the hydrocarbons is completely different in the pre-reforming case in that as the reactants are being mixed, a flammable mixture is produced. However, once mixing is complete there does not exist enough oxygen to produce a flammable mixture. Hence, combustion can be produced upon mixing, but there exists little danger of combustion once mixing is complete. Typically in such applications the oxygen is introduced as a high velocity jet designed to entrain the flammable gas quickly so that the flammable mixing zone is minimized. Flame arrestors can be also placed after the mixing zone to reduce the effect of overheating in the event the mixture accidentally ignites. These flame arrestors consist of a bundle of narrow passages that only permit axial flow.
A further problem in any reactor containing a catalyst is that eventually, the catalyst will have to be replaced. This can be a very arduous task that can take days to complete. In U.S. Pat. No. 4,865,820, an attempt is made to segregate the catalyst from insulation that serves to insulate the reactor walls from the high temperature reactions occurring within such reactor by provision of a reactor having an outer pressure vessel that contains insulation, an inner refractory and a metal sheath that contains the catalyst. The top mixing section can be removed to allow retrieval and reinstallation of the catalyst when requiring replacement. Even though the catalyst is formed of monolithic blocks, retrieving and reloading the catalyst is still problematical.
As will be discussed the present invention provides a catalytic reactor in which stable flame propagation within the mixing chamber is inhibited and is designed such that the catalyst can be easily installed and replaced.